Multi-beam lithography by means of electrically charged particles has been investigated since the 1980s. One important application of particle-beam lithography is in semiconductor technology. Therein, lithography apparatus are used to define structures on a target, e.g., a silicon wafer. (Throughout this disclosure, the terms target and substrate are used interchangeably.) In order to define a desired pattern on a substrate wafer, the wafer is covered with a layer of a radiation sensitive resist. Afterwards, a desired structure is imaged onto the resist by means of a lithography apparatus, and the resist is then patterned by partial removal according to the pattern defined by the previous exposure step and then used as a mask for further structuring processes such as etching. In another important application the structures on the target may be generated by direct patterning without a resist, for example ion milling or reactive ion beam etching or deposition.
In 1997, I. L. Berry et al., in J. Vac. Sci. Technol. B, 15(6), 1997, pp 2382-2386, presented a writing strategy based on a blanking aperture array and an ion projection system. Arai et al., U.S. Pat. No. 5,369,282, discuss an electron beam exposure system using a so called blanking aperture array (BAA) which plays the role of a pattern definition means. The BAA carries a number of rows of apertures, and the images of the apertures are scanned over the surface of the substrate in a controlled continuous motion whose direction is perpendicular to the aperture rows. The rows are aligned with respect to each other in an interlacing manner to that the apertures form staggered lines as seen along the scanning direction. Thus, the staggered lines sweep continuous lines on the substrate surface without leaving gaps between them as they move relative to the substrate, thus covering the total area to be exposed on the substrate.
Starting from Berry's concept, E. Platzgummer et al., in the U.S. Pat. No. 6,768,125, presented a multi-beam direct write concept dubbed PML2 (short for “Projection Maskless Lithography”), employing a pattern definition device comprising a number of plates stacked on top of the other, among them an aperture array means and a blanking means. These separate plates are mounted together at defined distances, for instance in a casing. The aperture array means has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures, wherein the apertures are arranged within a pattern definition field composed of a plurality of staggered lines of apertures, wherein the apertures are spaced apart within said lines by a first integer multiple of the width of an aperture and are offset between neighboring lines by a fraction of said integer multiple width. The blanking means has a plurality of blanking openings arranged in an arrangement corresponding to the apertures of the aperture array means, in particular having corresponding staggered lines of blanking openings. The teaching of the U.S. Pat. No. 6,768,125 with regard to the architecture and operation of the pattern definition device is hereby included as part of this disclosure by reference.
The PML2 multi-beam direct write concept allows for a large enhancement of the writing speed compared to single beam writers. This arises from the reduction of the required current density, the diminished importance of space charge due to the large cross section, the enhanced pixel transfer rate due to the parallel writing strategy, and the high degree of redundancy possible using a plurality of beams.
The U.S. Pat. No. 7,276,714 of the applicant/assignee discloses a pattern definition means for particle beam processing, comprising at least an aperture plate and blanking means. The apertures in the aperture plate are arranged in “interlocking grids”, i.e., the apertures are arranged in groups in squares or rectangles whose basic grids are meshed together. This means that the positions of the apertures taken with respect to a direction perpendicular to a scanning direction and/or parallel to it are offset to each other by not only multiple integers of the effective width of an aperture, as taken along said direction, but also by multiple integers of an integer fraction of said effective width. In this context, “scanning direction” denotes the direction along which the image of the apertures formed by the charged-particle beam on a target surface is moved over the target surface during an exposure process.
The “interlocking grids”-solution allows a finer resolution on the target surface even though the individual spots formed by each image of an individual aperture are not decreased in size. Particular values of the fractional offsets are integer multiples of ½K times the effective width of an aperture, where K is a positive integer.
Furthermore, the U.S. Pat. No. 7,276,714 describes the generation of grey scales by subsequent exposures of one pixel on the target by multiple apertures located in line, and shows how a shift register approach can be effectively applied to create grey scale patterns, i.e., exposure levels interpolated between a minimal (‘black’) and maximal (‘white’) exposure dose.
The state of the art PML2 concept is a strategy where the substrate is moved continuously, and the projected image of a structured beam delivers 100 percent of the exposure dose for the “white” pixels by subsequent exposures of apertures located in line. To realize grey levels, the total amount of apertures in line is subdivided into columns, the number of columns corresponding to the number of desired grey levels. In a recent variant described in the published US patent-application US 2008237460 by the applicant/assignee, a so called “trotting mode” writing strategy is proposed in which for each pixel one or a few beams along the (mechanical) scanning direction are used to deliver the full exposure dose to the pixels. The advantage of this variant is the reduced complexity of the CMOS structure and improved data management.
In all writing strategies based on a bitmap type of pattern coding, accurate placement control of the individual beams with respect to an ideal grid (physical grid, typically 20 nm pitch) is essential in order to fulfill lithographic requirements. While basically a systematic distortion of the image would cause a non-isotropic change of the blur, in the mentioned “trotting mode” a systematic distortion would also give rise to significant distortion of the pattern eventually generated since only one beam (or a few, depending on the chosen strategy and redundancy) contributes to the total exposure dose of one pixel. Similarly, the dose per level, which is directly related to the aperture size and current density at the respective position on the aperture plate, becomes more important in the case of the trotting mode.
Another parameter that has essential impact on the performance of the optical system is the current through the optical column of a maskless particle-beam exposure apparatus. The current is directly proportional to the charge density of the beam, i.e., the actual pattern density being exposed by the apparatus. The coulomb interactions of the particles (which depend on the charge density) are therefore dependent on the current. Thus, the blur of the individual beamlets increases with larger currents through the optical system. This blur, which is influenced by the coulomb interactions, is an essential parameter for the spatial resolution of the apparatus.
Especially when the exposure is performed in a scanning mode, the current will change when large features move into the exposure field. Thus, the target will be exposed with a strongly fluctuating blur at the feature borders, leading to a poor overall performance of the apparatus.
The same consideration applies when the apparatus uses grey-levels for exposing an image, as is described in the U.S. Pat. No. 7,276,714. Here, the current is not only dependent on the pattern density but varies, in addition, during the exposure process because of the grey levels being exposed.
Depending on the patterns to be exposed, the current variations may become as high as 100 percent of the maximum possible current, e.g., when a completely “white” exposure (all apertures open) follows a completely “black” exposure (all apertures closed).